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What are some typical and extreme values of thickness?

(see also "Thickness: what is it?": thanks to Jon O'Rourke for looking up the extreme values.)

As already noted elsewhere, the values of the (total) thickness between levels at 500hPa and 1000hPa give a useful measure of the mean temperature of that layer. In summer, values might range from 546dam (cool, showery northwesterly) to 560dam (warm, settled anticyclonic spell); in winter from 530dam( brisk, chilly, showery flow, with inland night frosts) to 550dam (mild, open-warm sector type). (The values are given for comparitive analysis only, and the weather types of course don't necessarily follow from the values); Values below 528dam in winter would herald the arrival of potentially wintry conditions, and in summer, thickness values above 564dam might be a precursor to some notably high temperatures.

I looked up some 1961-1990 average values for a couple of points across Britain (based on the RMetS 'Weather Log' charts & NOAA-CIRES/CDC Re-analysis project).

 

 [dekametres]

 

 Jan/Feb

 

 Jul/Aug

 Aberdeen

 

 531

 

 552

 London

 

 534

 

 557

 

For the period 1991-2005 (15 years) based on my own records, thickness values have risen (relative to the above & for a point roughly within the CET 'domain') by about 1.2 dam (representing roughly +0.6degC through the layer; 1997 and 2003 showed an increase on the 61-90 climatology of about +3dam/+1.5C. This accords well with expected changes due to anthropogenic global warming.

The increase noted above across 'Central England' is broadly confirmed by data released by the Met Office / Hadley Centre. They show that, relative to the 1961-1990 reference period, 'lower tropospheric' temperatures have increased by about 0.4degC averaged over the years 1991-2005.

As to extremes, for the UK mainland only, the highest Jon & I could find (from rather small-scale maps) came out around 575dam in July over southeast Kent (SE England), and the lowest around 495dam on the extreme tip of NE Scotland in January. However, for the British Isles, we have a low value of 491dam over Shetland, and values below 500dam can be recorded in exceptionally cold easterly types as far south as East Anglia & SE England; for example, in January 1987, total thickness values in these latter areas were certainly below 498dam, and probably briefly around 495dam. (Record low UK day-maxima on the 12th January, 1987). For a graphical representation of maximum and minimum thickness values for 6 points around the NW of Europe, see "Thickness Extremes".